The aim of Total Hip Arthroplasty (“THA”) is the reduction of pain by restoring the form and function of a hip joint damaged by either trauma or disease. This is accomplished using engineered materials to construct an implantable device for the restoration of the joint mechanics and geometry, whereby the affected tissue is removed and replaced by the implantable device. Successful outcomes depend largely on the proper sizing, placement and orientation of the implant. Incorrect biomechanics (e.g., joint reaction forces, soft tissue balancing, leg length, etc.) can slow or prevent healing, cause gait abnormalities, result in dislocation of the joint, and lead directly to early implant failure, among other things.
The restoration of proper joint mechanics depends largely on good surgical technique, implants which are anatomically matched to the needs of the patient, and effective instrumentation for bony preparation, size and shape determination, and insertion of the final implant construct.
Prior to insertion of the actual implant, it is generally desirable to use a mock implant or “trial” as a means of evaluating the correct size and positioning of the implant within the bony canal. The surgeon implants the trial, reduces the joint and evaluates the stability of the joint, leg length discrepancies, and range of motion (“ROM”). This process of “trialing” is often iterative as the surgeon tries different trial implants until the satisfactory joint mechanics are achieved.
The stability and range of motion of the hip joint is achieved by placing the prosthetic femoral head in an orientation with respect to the proximal end of the femur (i.e., the “proximal femur”) and knee joint that allows for a normal range of motion without impingement of the hip onto the acetabulum. This impingement can be either prosthetic or bony impingement. The tension of the joint, which provides stability, is achieved by adjusting the neck length of the implant. Neck length can be adjusted both vertically and horizontally, and is the distance between the center of the prosthetic head (i.e., the center of the acetabulum) and the centerline (or long axis) of the proximal femur. Increasing neck length increases the tension of the tendons and muscles that attach the proximal femur to the pelvis. The angular orientation of the neck of the femur with respect to the plane of the posterior condyles of the knee is defined as anatomic anteversion (FIG. 1).
The normal proximal femur has a gradual anterior twist to it so that the orientation of the endosteal envelope (i.e., inner bone geometry) gradually rotates externally (i.e., the head moves anteriorly relative to the transverse plane) in the proximal third of the femur until it culminates in the position of the femoral head. Thus the head of the femur is usually slightly more anteverted than the body of the femur with respect to the axis of the posterior condyles of the knee. The normal range of anteversion is about 20 degrees to about 30 degrees. However, various conditions can cause the natural anteversion to range from between about −20 degrees to about 50 degrees. When replacing the hip joint, the surgeon may need to change the patient's natural anteversion considerably so as to create proper and stable biomechanics of the hip joint.
Hip implants generally comprise four regions or sections: the head, the neck, the body, and the stem. The head section is almost always modular (i.e., detachable from the remainder of the implant). Most implants are one solid piece that comprises the neck, body, and stem regions. These one-piece hip implants are positioned inside the proximal femur in the orientation that best fits the body of the implant to the inside of the proximal femur (i.e., metaphysis). The neck of these devices either follows the orientation of the body exactly or is offset from the body by some fixed angle. Thus the neck of the device can be in only one orientation and the surgeon cannot change it to suit the particular anatomy of the patient. In many patients, this fixed orientation is sub-optimal and occasionally even insufficient, producing impingement or instability.
Modular implants can allow independent rotational positioning of the neck with respect to the body in the transverse plane, thereby providing the surgeon with the ability to solve the angular positioning issues of hip replacement surgery. Several types of these modular implants exist. However, with all currently available devices, the surgeon must visually estimate the correct orientation of the implant neck during the trial phase of the surgery. Anatomic landmarks such as the orientation of the proximal femur are most often used to gage anteversion. The surgeon essentially “eyeballs” the anteversion angle relative to the partially visible proximal femur. However, the surgeon has no idea what the actual mechanical anteversion angle is relative to the proximal femur. Often the surgeon makes a mark on the adjacent tissue to indicate the approximate orientation of the trial neck relative to the trial body. If the orientation of the trial neck and body thereafter has to be changed, the surgeon iteratively rotates the neck by some approximated angular amount to another position, until the leg mechanics seem correct. After the trial process is completed, the surgeon selects the appropriate implant components and inserts them into the bone. The surgeon must once again estimate the orientation of the neck relative to the body, hoping to achieve the same position that was achieved with the trial. This process often results in the surgeon settling for a “that looks about right” orientation.
No current instrument or method allows the surgeon to actually measure the anteversion of the prosthetic neck with respect to some reference frame.
No current instrument or method allows the surgeon to insert the implant neck into precisely the same orientation as was chosen in the trial step.